Chemically milling titanium alloy workpieces

ABSTRACT

APPARATUS AND ETCHANT COMPOSITIONS ARE DISCLOSED FOR CHEMICALLY MILLING TITANIUM ALLOY WORKPIECES TO DEVELOP RECESSED AREAS HAVING NON-UNDERCUT EDGE CHARACTERISTICS.

July l0, 1973 D, O, CQWLES ET AL 3,745,079

v CHEMICALLY MILLING Tl'IANIuM ALLOY WORKPIECES Filed April 2l. 1971 WILLIAMA l.. HACK ATTORNEY Unted AStates Patent Office 3,745,079 Patented July 10, 1973 3,745,079 CHEMICALLY MILLING TITANIUM ALLOY WORKPIECES Donald O. Cowles and William L. Hack, Tulsa, Okla., assignors to North American Rockwell Corporation Filed Apr. 21, 1971, Ser. No. 136,020 Int. Cl. C23g 1/06, 1/22 U.S. Cl. 156-18 2 Claims ABSTRACT OF THE DISCLOSURE Apparatus and etchant compositions are disclosed for chemically milling titanium alloy workpieces to develop recessed areas having non-undercut edge characteristics.

SUMMARY OF THE INVENTION Structural members such as aircraft skin components and the like with recessed surface areas are chemically milled from titanium alloy stock using essentially conventional apparatus and an etchant consisting of hydrouoric acid solution, wetting agent, and ammonium nitrate etch control additive to obtain non-.undercut recessed area edges and other improved workpiece characteristics. The configuration and slope of the recessed area edge may be varied within limits by varying the concentration of etch control additive in the etchant.

DESCRIPTION OF THE DRAWINGS The apparatus illustrated schematically in FIG. 1 is preferred for use in connection with the practice of the present invention As shown in that figure, a titanium alloy aircraft skin workpiece 11 is typically supported vertically in open basket 12 for purposes of removing metal from selected surface areas by chemical milling. Basket 12 is suspended by straps 13 from reciprocally moved support member 14. Both workpiece i11 and basket 12 are immersed in etchant solution 15 and mechanical agitation of member 14 within bearing supports '16 and etchant-resistant tank 17 is accomplished by conventional vibrator unit 18 supported on platform 19.

Apparatus also preferably includes a refrigeration unit 20 that functions to control the temperature of etchant by controlled circulation of a coolant through immersed cooling coil 21. An air supply 22 cooperating with valve 23 and manifold 24 is provided to supply pressurized air to etchant 15 through inlets 25 and thereby obtain an additional degree of etchant agitation and circulation relative to the exterior surfaces of workpiece 11.

In one embodiment of apparatus 10, refrigeration unit 22 was typically operated to control the temperature of etchant 15 to a temperature in the nominal range of 90 F. to 130 F. with a temperature of approximately 115 F. normally being preferred. Vibrator unit 18 developed reciprocal transverse movement of support member 14 at a velocity of l2 inches per second on the average, such rate typically being in the normally preferred velocity range of from 6 inches per -second to 12 inches per second. Variation of etchant temperature,

and to a lesser degree agitation rate, noticeably affects the rate of metal removal from workpiece 11.

The etchant preferredin the practice of the present invention for removing metal from alloys such as 6Al-4V titanium alloy is comprised of hydrofluoric acid water solution, an etch control additive such as ammonium nitrate, and a wetting agent. By way of example, one specific preferred composition of etchant 15 is: 92.5 gallons water; 7.5 gallons concentrated hydrouoric acid (commercial grade); 0.94 pound ammonium nitrate (commercial grade); and milliliters dodecylbenzene sulphonic acid wetting agent. As will hereinafter be detailed, the preferred amount of ammonium nitrate etch control additive may be varied over the range of from approximately 0.50 pound per 100 gallons of etch solution to 1.30 pounds per 100 gallons of etch solution to obtain different edge cross-section configurations and slopes at the perimeter of chemically milled recessed surface areas in titanium alloy workpieces.

FIG. 2 illustrates workpiece 11 in cross-section and shows a removable conventional etch-resistant film 26 adhered to the workpiece surface. Film 26 is typically comprised of a conventional polyvinyl chloride and is applied to workpiece 26 by spraying or dipping. After the coating is cured on the workpiece, surface areas from which metal is to be removed are dened by edges 27 scribed in the etch-resistant film. Inset film areas are typically -stripped manually from the workpiece surface to expose the surface metal of workpiece 11 throughout areas from which metal is to be removed.

Conventional chemical milling etchants having esentially only water, hydrouoric acid, and wetting agent function to remove surface metal from titanium alloy workpieces in a manner that produces an undercut or fingernail cross-sectional configuration at recess edges of workpiece 28 and iilm edge 27 as shown in FIG. 3. We have discovered that incorporating ammonium nitrate etch control additive in etchant 15, generally in the range from approximately 0.5 0 pound per 100 gallons of etchant to 1.30 pounds per 100 gallons of etchant (based on an etchant equivalent of 7.5% volumetric concentration of hydrofluoric acid in water), is eifective to eliminate such fingernail effect and to produce edge configurations of the type shown in FIGS. 4 and 5. Such latter type is generally acceptable in connection with producing aircraft quality chemically milled titanium alloy workpieces. The smooth recess fillet in workpiece 29 at iilm edge 27 (having a departure from normal of essentially 0 at the workpiece upper surface) is produced by practice of the instant invention if the amount of etch control additive included in etchant 15 is approximately 0.50 pound per 100 gallons of etchant with 7.5% hydrouoric acid volumetric concentration in water. By increasing the amount of ammonium nitrate etch control additive to approximately 1.00 pound per 100 gallons of etchant with 7.5% hydrofluoric acid volumetric concentration in water, the slope condition shown in workpiece 30 of FIG. 5 may be obtained at film edge 27. Such recess edge slope is approximately 45 and is normally obtained over the range of approximately 35 to 45 by the addition of from approximately 0.75 pound to 1.00 pound of ammonium nitrate etch control additive to the specified concentration of hydrouoric acid etchant. When the quantity of etch control additive is increased to approximately 1.30 pounds per 100 gallons of specified etchant solution, the slope shown in FIG. 5 may be changed to a condition where angle A equals approximately For most aircraft quality work a recess edge slope of 35 to 45 is preferred.

Etchant solution 15 provides a smoother chemically milled edge in the titanium alloy workpiece when initially containing a small proportion of dissolved alloy material. For this reason we recommend initially including approximately 2.0 pounds of dissolved titanium alloy in the' etchant 15 (per 100 gallons) prior to initiating chemical milling of any titanium all-oy workpiece. When the concentration of dissolved metal in the etchant reaches a level of approximately 10 to 12 ounces of dissolved metal per gallon of solution, it normally becomes necessary to replace the etchant or provide for adequate reconstitution.

We claim:

'1. In a method of chemically etching a partiallymasked titanium alloy workpiece containing aluminum and vanadium, the step of removing metal by chemical reaction between said titanium alloy workpiece and a solution consisting of approximately 4% by weight hydrogen fluoride, ammonium nitrate in the range of 0.06% to 0.5% by weight, and the balance water and wetting agent to eliminate undercutting at the workpiece metal edges adjacent workpiece unmasked areas.

2. The invention defined by claim 1 wherein said ammonium nitrate is provided in said solution in the range of approximately 0.09% to 0.12% by weight.

References Cited UNITED STATES PATENTS 

